The present invention relates to sheet support and guiding arrangement in general, and more particularly to such arrangements for use in printing machines which print images on both major surfaces of sheets passing therethrough.
Printing machines of the above-mentioned type are already known and in widespread use so that they need no detailed discussion. Suffice it to say that they include a plurality of printing stations which print images first on one side of the respective sheet and then on the other side. Thus, the respective sheet is to be turned or reversed as it passes from the printing station or printing stations which print images on one of the sides or major surfaces of the respective sheet, to the printing station or printing stations which print images on the other side or major surface of such sheet. Reversing arrangements for turning the sheets are also already known, among them such which work according to the principle of turning the trailing end of the respective sheet. During or following the turning of the respective sheet, and during the subsequent advancement of the respective sheet from one of the printing stations to the next following one, the major surface of the respective sheet which has a fresh printed image thereon faces downwardly so that, when the downwardly facing major surface, and particularly the fresh image printed thereon, is allowed to come into contact with sheet-guiding arrangements, there exists the danger that the fresh image may become smeared or blurred due to the sliding contact of the image with the sheet guide arrangements or elements during the relative movement of the respective sheet with respect to the guide arrangements.
In an attempt to avoid this danger, it has been already proposed to use, in printing machines of the type printing images on both major surfaces of the sheets, such arrangements for supporting and guiding the respective sheets which include hollow sheet-guiding brackets arranged at the printing stations, through and from which air is blown underneath the advancing sheet so that the sheet is suspended above the sheet-guiding brackets by the air issuing from the latter, following the turning of the sheet, in order to avoid smearing of the freshly printed image.
This proposed arrangement is disadvantageous in several respects. First of all, a pronounced disadvantage is that too much of the air being blown against the sheet escapes underneath the sheet so that there is no chance of formation of an air cushion for supporting the sheet. In order to achieve the desired effect nevertheless, considerable amounts of air must be blown out of the outlet apertures of the sheet-guiding brackets, which amounts of air can be produced only at a great expenditure in terms of capital investment and operating expenses. An additional disadvantage of this proposed arrangement is that the substantial amount of air blown against the sheet brings about the danger that the marginal portions of the sheet will be turned over and, during the subsequent printing operation, the turned-over portions of the sheet will be fixed in their positions, resulting in the formation of folds and creases on the sheet. A further disadvantage of this arrangement is to be seen in the fact that sheets of heavy paper cannot be treated in a printing machine equipped with such an arrangement because of their high unit weight, in that such sheets, despite the localized air stream, "stall" or drop onto the sheet-guiding brackets and thus the images printed on the downwardly facing major surface of the respective sheet become smeared or obliterated. Furthermore, the advancement of the sheets during the printing on the first side of the sheet is unsteady due to the absence of closed guiding baffles.